Not applicable.
This application includes a one-disk CD-R Appendix, the full contents of which are incorporated by reference herein. The disk contains the following files, listed by file name, creation date and file size in bytes: xe2x80x9cColorBlind Video Startupxe2x80x9d created Jun. 22, 1999, 100K; xe2x80x9cColorBlind_Video_Startup.exexe2x80x9d created Sep. 16, 1999, 160K; xe2x80x9cComplete_Text.txtxe2x80x9d created Feb. 22, 2001, 40K; xe2x80x9cContents_of_CD.txtxe2x80x9d created Feb. 22, 2001, 4K; xe2x80x9cLab Color Space Profilexe2x80x9d created Aug. 4, 1997, 7K; xe2x80x9cProve it!xe2x80x9d created Aug. 27, 1999, 4,116K; xe2x80x9cProve it! Installerxe2x80x9d created Jul. 20, 1999, 2,251K; xe2x80x9cProve it_Beta.exexe2x80x9d created Jul. 16, 1999, 11,555K; and xe2x80x9cProve_it_Setup.exexe2x80x9d created Nov. 18, 1999, 7,432K.
This invention relates to the calibration of computer monitor displays, more particularly to a method of monitor calibration using target set screen displays and no supplemental light-measuring instrument.
The invention makes possible a new and superior method for calibrating visual display devices. When using computer systems to view, control and/or print graphics or photographs, it is often critically important that the computer displays be calibrated to a chosen standard condition.
Once a display (also called a xe2x80x9cmonitor,xe2x80x9d particularly when referring to computer displays) is calibrated to a known state, techniques can then be employed to cause the display to accurately simulate the appearance of an image or graphic as it appears on, or when printed from, or as seen by another digital imaging device. Absent correct calibration, such simulations become inaccurate in like degree. The capability for accurate simulation is of tremendous importance to digital imaging in general. Therefore, display calibration is an important issue for many people.
The current invention can also be applied to the calibration of television set displays and all other types of analog or digital displays having many levels of intensity in each color channelxe2x80x94typically three channels, one each for red, green and blue.
The current invention is embodied in commercially available software for display calibration called ColorBlind Prove it! from ITEC Color Solutions, San Diego, Calif. The software exists in both Macintosh and Windows versions.
The prior art makes visual (instrumentless) calibration possible, but in no case does it teach a complete method for obtaining high-quality calibration. The prior art furthermore fails to address a variety of significant problems inherent in display calibration.
The Knoll Gamma version 2.0 application (see FIG. 2), published by Adobe Systems Incorporated of San Jose, Calif., provides an incomplete system, which is capable of visual calibration of low to moderate quality.
U.S. Pat. No. 5,298,993 by Albert Edgar and James Kasson teaches certain useful principles of visual calibration, including the use of targets.
U.S. Pat. No. 5,638,117 by Peter Engledrum and William Hilliard uses areas of parallel lines in a visual characterization process.
The Default Calibrator application from Apple Computer, Inc. of Cupertino, Calif. (see FIG. 3), which is a part of the program entitled ColorSync 2.5 and later, teaches a crude method for display calibration. This is prior art for only that part of the present invention called Gray Balance Method One (see FIG. 11, xe2x80x9cGray Balance Method One Targetxe2x80x9d). The three lined blended-region-and-solid sub-targets of FIG. 3 are red, green and blue, from left to right.
The image file xe2x80x9cGamma_Estimationxe2x80x9d (see FIG. 4), from Candela, Ltd. (now Pictographics International Corporation) of Burnsville, Minn., teaches a method of identifying the current overall gamma from a broad range of possible overall gammas with the use of a gradient.
The image file xe2x80x9cCurrent Gammaxe2x80x9d (see FIG. 5), published by Adobe Systems Incorporated of San Jose, Calif., in the program entitled PageMaker 6.5, teaches another method to identify the current overall gamma from a broad range of possible overall gammas.
The image file xe2x80x9cGamma 1.8.tifxe2x80x9d (see FIG. 6), also included with Adobe PageMaker version 6.5 software, from Adobe Systems Incorporated of San Jose, Calif., together with Knoll Gamma 2.0.1 and an explanatory text file entitled xe2x80x9cGamma Read Me,xe2x80x9d teaches an improvement in the accuracy of the verification of a fixed gamma. However, xe2x80x9cGamma 1.8.tifxe2x80x9d has significant limitations with regard to its ability to reliably reveal the correct gamma within each of several distinct subsegments of the tone scale because of the way in which tones were chosen to construct the target and the limited number of sub-targets used. xe2x80x9cGamma 1.8.tifxe2x80x9d looks at six regions of the tone scale which all overlap a great deal and therefore can obscure the true nature of any observed departure from the gamma curve being sought, thus hindering efficient adjustment to achieve ideal gamma 1.8 tonality. Like all visual gamma adjustment targets, xe2x80x9cGamma 1.8.tifxe2x80x9d also cannot be used to verify conformity of any display to other gamma curves, such as 2.2. The target of FIG. 6 comprises various lined blended-region-and-solid sub-targets each having a repeating pattern comprising the alternation of a single lighter row with a single darker row (each row being the same height) juxtaposed with a gray solid region of a value between the lighter and darker values.
The standard condition to which a display is calibrated is defined partially by the inherent colors, or chromaticities, of the display""s purest red, green and blue colors. In the case of a common CRT (cathode ray tube) type display, these colors of red, green and blue are determined primarily by the colors, or chromaticities, of the phosphors used in the tube. In the case of a flat panel display (FPD), these colors are determined by the mechanism of the display which created the primary colors, which is always different from that of CRTs.
The three other principal aspects of display calibration are not fixed by the nature of the hardware itself (that is, the display or monitor). These three other aspects of display calibration are:
1) Calibrating the white point, i.e. the color of the display""s white (independent of its brightness), also known as its Hue and Chroma, also known as its x,y coordinates from the X+Y+Z=1 plane of the CIE XYZ color space;
2) Calibrating its gray balance so that each gray that it displaysxe2x80x94from black (the darkest color the display can display in a given state of calibration) all the way to white (the brightest color the display can display in a given state of calibration)xe2x80x94has the same color as the white (also known as the same Hue and Chroma, also known as the same x,y coordinates); and
3) Calibrating the xe2x80x9cgammaxe2x80x9d or tone curve of the display so that the way it displays the full range of input values from black to white follows the desired progression of luminous intensities. Typically, the full range of digital input values sent from the computer""s video circuitry to the display is the range from RGB (0, 0, 0) to RGB (255, 255, 255) where each color channel has a range of 256 (two to the eighth power) values. Gamma curves are a subset of all possible tone curves and have certain mathematical properties. Displays generally need to be calibrated to a gamma curve to function properly as a calibrated display in a color managed system of imaging devices.
Two other aspects of display calibration are:
1) Calibration of absolute white intensity; and
2) Calibration of absolute black intensity.
In addition to the aspects of display calibration mentioned above, there are particular adjustments of so-called hardware controls, such as the Brightness, Contrast, Color, Bias, and Gain controls on CRT displays, which adjustments affect and/or are part of the processes of calibration mentioned above. The affected processes include calibration of the white point, the gray balance, the gamma, the absolute white intensity, and the absolute black intensity.
Flat panel displays (FPDs) exhibit different natural tone curves and white points from CRTs, and typically have different kinds of hardware controls, which affect the appearance of data displayed by these displays. FPDs also require calibration, for essentially the same reasons that CRT displays do.
Display calibration techniques in the prior art can be broken down into two main categories of calibration.
One such category is instrumented calibration, where many steps in the whole process of calibrating a display are carried out by attaching a photometer, colorimeter or spectrophotometer to the surface of the display. The attendant display calibration software causes a variety of colors to be displayed by the display. Color measurement readings are taken by the colorimeter (or other instrument), and the software utilizes these readings to facilitate most or all aspects of the calibration process.
The other main category of display calibration is visual calibration, which is accomplished without a photometer, colorimeter, spectrophotometer, or spectroradiometer for the measurement of the display and instead relies on a variety of techniques, methods and processes to accomplish essentially the same things as the instrumented calibration processes. Central to the visual calibration processes are visual targets which provide visual feedback regarding the state of the display, which then allow the user to make informed adjustments to the display using both the built-in display controls and visual calibration software. The visual calibration software modifies the video card LookUp Tables (LUTs) in response to user adjustments of sliders and like on-screen software controls. The software may also perform a variety of related functions such as automated sequential presentation of the visual calibration targets and their attendant tool interfaces, presentation of user instructions, and user education in relevant matters.
Each type of calibration, instrumented and visual, can be implemented with widely varying degrees of success, and the visual methods are potentially and generally more economical due to the lack of the need for a light measuring instrument. The present application concerns itself primarily with visual calibration and not with instrumented calibration.
Both types of calibration can be accomplished with a myriad of variations in the exact details of implementation and methods, but the prior art in visual calibration is inadequate in several key respects, which frequently makes it inadequate for high-quality work. This is due to limitations on its ability to detect and to overcome inherent problems with the nature of the display hardware""s behavior and also due to limitations in the ability of the user to have confidence in the accuracy of a calibrated state achieved by prior art visual calibration methods. These problems are solved by the present invention. To a significant degree, the same problem of user uncertainty about the accuracy achieved by instrumented calibration methods also exists and is solved by the present invention.
The present invention relates to a novel process of visual calibration of computer displays. The current invention addresses all of the significant shortcomings of the prior art and, for the first time, provides a method for complete, high quality calibration of displays, particularly all computer displays. It successfully addresses each of the five specific problems in the prior art.
First, the current invention provides an objective visual method for determining the precisely optimal brightness setting for CRT displays, which method is also applicable to the setting of the xe2x80x9cBlack Levelxe2x80x9d control on some FPDs. Accurate brightness or black level setting or its equivalent tone control in the video card LUTs is a requirement for achieving any of the necessary standard tone curves. Prior art solutions to setting the brightness gave vague and subjective assessments of the shadow tonalities, which derive from the range of possible brightness settings. By finding a certain tonal relationship between two adjacent or slightly overlapping tone regions close to black, the desired curve shape can be found, by user adjustment of the brightness control while looking at the target, which is appropriate to the level of flare in their system. The system includes at least the hardware, the video LUTs and the viewing environment.
Second, the current invention provides a precise method to visually determine conformity of a display""s tonality to a given standard tone curve, for example a gamma 1.8 curve, which is embodied in one of the preferred calibration target sets of the invention. The current invention clearly reveals conformity with the standard tone curve for each relevant sub-region of the tone scale, thus assuring a visual tone match between the displayed image and the image data when simulations are performed (see FIGS. 13-17). The prior art methods provide only limited ability to verify the actual tonality of a display and its conformity to a standard tone curve, because the methods of the prior art do not reveal the tonality of each relevant subsection of the tone curve. Also, some prior art methods rely on a gridded or halftone pattern of mixed dark and light tones instead of a pattern of alternating horizontal lines, rendering them essentially useless for reliable tone calibration of CRT displays because of limitations of the electronics of CRT displays. The present invention relies primarily on patterns of mixed light and dark tones, which consist of horizontal lines containing only one value, for gamma or tonality assessment. The invention relies primarily on such lined patterns, which complement the nature of currently ubiquitous display hardware used for imaging. The invention also makes it feasible to implement a solution which, in the visual calibration targets, includes the mixing of pixels of different values in individual, horizontal rows of pixels, especially in the Gray Balance Method Two procedure and target. Flat panel displays, which are not in widespread use for imaging, are likely to be much better suited to use with such mixed pixel values in individual horizontal rows of pixels in the visual calibration targets than are CRT displays of the present day. The invention also makes it possible to combine the gamma and gray balance adjustment functions into a single target set which relies on a combination of patterns such as described below in the Gray Balance Method Two targets (see FIGS. 9 and 23).
Third, the current invention is unique in meeting the need to sense and therefore be able to control and to verify the correct gray balance of the entire tone scale of the display. A process of visual comparison of the color (Chroma and Hue, or x,y coordinates) of each major region of the tone scale to that of the white is provided. The prior art at best only facilitated visual assessment of the gray balance of a portion of the tone scale. Side-by-side comparison of most of the colors in question is used, with each seen at the same lightness, instead of highly disparate lightnesses. Most importantly, a pattern of alternating tones arranged in lines is used which extends the ability to view adjacent, same-lightness gray regions not only to the midtones or upper midtones, but to the three-quartertone and quartertone regions of the tone scale as well. A second preferred embodiment extends this reach to a darker value still, essentially covering the entire tone scale. The three-quartertone, midtone and quartertone regions are not merely the regions of 25%, 50% and 75% luminance, rather, they are the more widely spaced regions of roughly 25%, 50% and 75% of the zero to 255 RGB input values scale of a display calibrated to a common gamma standard, such as 1.8 or 2.2 (see FIG. 9, illustrating what is referred to herein as Gray Balance Method Two, the first of two preferred embodiments of this part of the invention). In each of two preferred embodiments shown, a gray scale is added to the three or four lined-versus-solid sub-targets to augment the ability to sense and correct gray balance errors near black and to verify the approximate gray balance of the entire tone scale. In the case of a display calibrated to a gamma of 2.2, the absolute luminosity of the midtone is roughly 25% to 30% of that of the white and the absolute luminosity of the three-quartertone is roughly 6% of that of the white.
The second preferred embodiment of the gray balance capability of the invention (see FIGS. 23, 24 and 25, illustrating what is referred to herein as Gray Balance Method Two, Increased Precision) extends the ability to see even further into the shadows by utilizing a pattern of dots, at the risk of making this embodiment function less ideally with CRT displays, in terms of correct lightness in the blended areas. This embodiment allows double the gray balance matching precision in the three-quartertones, relative to the first embodiment, and brings precise matching to an even darker value of RGB (40, 40, 40) in this target example designed for use at gamma 2.2. This is more than 5/6ths of the way from white to black in the input value scale, and is a tone which, in this case of a gamma 2.2 version of the target, equals only 2% of the absolute luminosity of the white. This second preferred embodiment also increases the precision of the quartertone matching when used with displays having color crossovers in the lighter half of the tone scale. Flat panel displays are more prone to this problem than are CRT displays. Numerous variations on this part of the invention are possible, as necessitated by the nature of the display, the nature of the software interface, or the need for simplicity in the method. For example, more sub-targets can be added at almost any lightness where user control is needed to force correct tonality or gray balance. Also, a single target set of a type broadly similar to the Gray Balance Method Two, Increased Precision target (see FIGS. 23, 24 and 25) can be used for both gamma and gray balance adjustment. By combining tones in new and more complex patterns than the prior art, the invention makes it possible to extend gray balance from the white to a midtone value, and from there to other values, and so on, until all important parts of the tone scale have been reached with the ability to match them to the color of the white.
The same extension of the user""s ability to see into the three-quartertones and quartertones provided by the current invention as seen in FIG. 9, is found in the secondary gray balance method of the current invention (see FIG. 11, illustrating what is referred to herein as Gray Balance Method One). In this case the user is taught to rely on matching lightness instead of matching Chroma and Hue, because the target splits the image into its separate red, green and blue channels (see sub-target rows 2101, 2102 and 2103 of FIG. 11).
Fourth, to solve the problem of verifying the similarity of the tone curve in the display profile and that of the actual calibration, a new method is presented in the current invention. By converting a preferred RGB gamma target, such as that discussed above and shown in FIG. 13, into CIE Lab image data through an ideal display profile of the correct gamma, a new kind of target is created. This Lab gamma target has an identical appearance to that of the original RGB gamma target if and only if the transformation through a selected display profile from Lab back to RGB is the precise reverse of the original transformation. Deviations in the appearance of the RGB and Lab gamma targets, seen with the aid of soft-ware created to implement the targets and methods of the present invention, reveal any mismatch between the profile and the calibration that affects the grayscale of an image.
Fifth, the above features of the current invention make the use of a visual calibration method based on most or all of the above features a practical, realistic, economical, and highly effective solution to high-quality display calibration. Even the sum of all prior art does not teach a complete solution to high-quality visual display calibration, and so visual calibration has always been relegated to a strictly second-class role. Now visual calibration can be of such high quality as to be the best method to verify instrumented calibration success or failure under most circumstances.
To complete the process of making a display ready for imaging work, a profile must be made or obtained which complements the calibration. When instrumented display calibration is performed, it is typical of software used for this purpose to also make a profile from information about the display obtained by measurement. For the visual approach to work well, the remaining information which describes the correct absolute color of the display white and the display""s pure red, green and blue must be obtained. Fortunately, all four of these numerical values can readily be obtained with sufficient precision to complete the process of readying a display for performing high-quality simulations and color matching, without the necessity of user measurement of the display.
Other novel features which are characteristic of the invention, as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawing, in which preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawing is for illustration and description only and is not intended as a definition of the limits of the invention. The various features of novelty that characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. The invention resides not in any one of these features taken alone, but rather in the particular combination of all of its structures for the functions specified.
There thus has been broadly outlined the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form additional subject matter of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based readily may be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.
Further, the purpose of the Abstract is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is neither intended to define the invention of this application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way.
Certain terminology and derivations thereof may be used in the following description for convenience in reference only, and will not be limiting. For example, words such as xe2x80x9cupward,xe2x80x9d xe2x80x9cdownward,xe2x80x9d xe2x80x9cleft,xe2x80x9d and xe2x80x9crightxe2x80x9d would refer to directions in the drawings to which reference is made unless otherwise stated. Similarly, words such as xe2x80x9cinwardxe2x80x9d and xe2x80x9coutwardxe2x80x9d would refer to directions toward and away from, respectively, the geometric center of a device or area and designated parts thereof. References in the singular tense include the plural, and vice versa, unless otherwise noted.